Shallow trench isolation structures

ABSTRACT

Shallow trench isolation structures are provided for use with UTBB (ultra-thin body and buried oxide) semiconductor substrates, which prevent defect mechanisms from occurring, such as the formation of electrical shorts between exposed portions of silicon layers on the sidewalls of shallow trench of a UTBB substrate, in instances when trench fill material of the shallow trench is subsequently etched away and recessed below an upper surface of the UTBB substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.13/531,654, filed on Jun. 25, 2012, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The field generally relates to STI (shallow trench isolation) structuresand, in particular, STI structures for UTBB (ultra-thin body and buriedoxide) semiconductor devices.

BACKGROUND

With SOI (silicon-on-insulator) technology, a thin silicon layer isformed over an insulating layer, such as silicon oxide, which in turn isformed over a bulk substrate. The insulating layer is referred to as aBOX (buried oxide) layer. For a single BOX SOI wafer, the thin siliconlayer is divided into active regions using STI structures, whichintersect the BOX layer. In general, the STI structures are fabricatedby etching a pattern of trenches in the SOI substrate below the BOXlayer, and depositing one or more layers of dielectric material to fillthe trenches. The STI structures define the active regions, and provideisolation between active regions in the upper silicon layer of the SOIin which devices such as FETs (field effect transistors) are formed. Thegate structures of FETs are formed on top of the thin silicon layer, forexample, by depositing a gate dielectric layer and a gate electrodelayer on the top surface of the thin silicon, followed byphotolithographic patterning, and etching to form gate stack structures.The sources and drains of field effect transistors are then formed, forexample, by ion implantation of N-type and/or P-type dopant materialinto the thin silicon layer using a gate stack structure to self-definea channel region.

In a single BOX SOI structure, the silicon layer underlying the BOXlayer can be used as a back gate layer under the active regions, whereinthe BOX layer serves as the back-gate dielectric, wherein the back gatecan be defined by either P+ or N+ implantation. FETs with back gatestypically use a UTBB (Ultra-Thin Body and Box) substrate having arelatively thin upper silicon layer and thin BOX layer to enable fullydepleted device operation with a threshold voltage that is responsive toa back gate voltage applied to the back gate (lower silicon layer). FETsfabricated using UTBB technology with back gates have significantadvantages such as, for example, reduced short channel effects, lessthreshold variability due to body doping fluctuations, and ability touse the back gate voltage to adjust the threshold voltage

However, as the thickness of the BOX layer is reduced for UTBBstructures, the potential for electrical shorts between the upper andlower silicon layers of the device increases as a result of variousprocessing steps that can etch down the trench fill material and exposethe upper and lower surfaces of the upper and lower silicon layers onthe sidewalls of the shallow trench.

SUMMARY

Aspects of the invention include STI (shallow trench isolation)structures and, in particular, STI structures for use with UTBB(ultra-thin body and buried oxide) semiconductor substrates, whichprevent defect mechanisms from occurring, such as the formation ofelectrical shorts between exposed portions of silicon layers on thesidewalls of shallow trench of a UTBB substrate, in instances whentrench fill material of the shallow trench is subsequently etched awayand recessed below an upper surface of the UTBB substrate.

In one aspect of the invention, a semiconductor device includes asemiconductor substrate having a first silicon layer, a second siliconlayer, and a buried oxide layer disposed between the first silicon layerand the second silicon layer, a high-k gate dielectric layer formed onthe first silicon layer, and a shallow trench isolation structure formedin the semiconductor substrate. The shallow trench isolation structureincludes a shallow trench formed through the first silicon layer, theburied oxide layer and partially through the second silicon layer, afirst liner conformally lining the shallow trench, and a trench fillmaterial disposed in the shallow trench. The first liner is formed of amaterial having etch selectivity with regard to the trench fillmaterial.

In another aspect, an upper sidewall portion of the shallow trenchisolation structure includes a void region disposed between an uppersurface of the first liner and an upper surface of the first siliconlayer, wherein the void region isolates the first liner from the high-kgate dielectric layer formed on the first silicon layer.

In another aspect of the invention, a semiconductor device includes asemiconductor substrate comprising a first silicon layer, a secondsilicon layer, and a buried oxide layer disposed between the firstsilicon layer and the second silicon layer, a high-k gate dielectriclayer formed on the first silicon layer, and a shallow trench isolationstructure formed in the semiconductor substrate. The shallow trenchisolation structure includes a shallow trench formed through the firstsilicon layer, the buried oxide layer and partially through the secondsilicon layer, a first liner conformally lining the shallow trench, anda trench fill material disposed in the shallow trench. The first lineris formed of a material having etch selectivity with regard to thetrench fill material, and the first liner is recessed below an uppersurface of the first silicon layer on the upper sidewalls of the shallowtrench. Further, an insulating material is disposed on the uppersidewalls of the shallow trench between the first liner and the uppersurface of the first silicon layer. The insulating layer isolates thefirst liner from the high-k gate dielectric layer.

In yet another aspect of the invention, a method of forming asemiconductor device includes forming a high-k gate dielectric layer ona semiconductor substrate having a first silicon layer, a second siliconlayer, and a buried oxide layer disposed between the first silicon layerand the second silicon layer, and forming a shallow trench isolationstructure foamed in the semiconductor substrate. The process of forminga shallow trench isolation structure includes forming a shallow trenchin the substrate through the first silicon layer, the buried oxide layerand partially through the second silicon layer, forming a first linerconformally lining the shallow trench, and filling the shallow trenchwith a trench fill material. The first liner is formed of a materialhaving etch selectivity with regard to the trench fill material. Inanother aspect, forming the shallow trench isolation structure furtherincludes etching the first liner to recess the first liner down in theshallow trench and create a void region disposed between an upperrecessed surface of the first liner and an upper surface of the firstsilicon layer, wherein the void region isolates the first liner from thehigh-k gate dielectric layer formed on the first silicon layer.

In another aspect of the invention, a method of forming a semiconductordevice includes forming a high-k gate dielectric layer on asemiconductor substrate having a first silicon layer, a second siliconlayer, and a buried oxide layer disposed between the first silicon layerand the second silicon layer, and forming a shallow trench isolationstructure formed in the semiconductor substrate. The process of forminga shallow trench isolation structure includes forming a shallow trenchin the substrate through the first silicon layer, the buried oxide layerand partially through the second silicon layer, forming a first linerconformally lining the shallow trench, and filling the shallow trenchwith a trench fill material, etching the first liner to recess the firstliner down in the shallow trench and create a void region disposedbetween un upper recessed surface of the first liner and an uppersurface of the first silicon layer, and filling the void region with aninsulating material. The first liner is formed of a material having etchselectivity with regard to the trench fill material, and the insulatingmaterial isolates the first liner from the high-k gate dielectric layerformed on the first silicon layer.

These and other aspects and features of the present invention willbecome apparent from the following detailed description of preferredembodiments thereof, which is to be read in connection with theaccompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor device having a STIstructure according to one aspect of the invention.

FIG. 2 is a cross-sectional view of a semiconductor device having a STIstructure according to another aspect of the invention.

FIG. 3 is a cross-sectional view of a semiconductor device having a STIstructure according to another aspect of the invention.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, 4J, 4K, 4L, and 4Mschematically illustrate methods for fabricating a semiconductor devicehaving STI structure according to aspects of the invention, wherein:

FIG. 4A is a cross-sectional view of a semiconductor device at aninitial stage of fabrication wherein a substrate comprises a firstsilicon layer, a second silicon layer, and a BOX layer disposed betweenthe first and second silicon layers,

FIG. 4B is a cross-sectional view of the structure of FIG. 4A afterforming a pad oxide layer on the substrate and forming a pad nitridelayer on the pad oxide layer,

FIG. 4C is a cross-sectional view of the structure of FIG. 4B afterforming a photolithographic mask on the pad nitride layer which definesan opening for etching a shallow trench,

FIG. 4D is a cross-sectional view of the structure of FIG. 4C afteretching the pad nitride and oxide layers and substrate to form a shallowtrench,

FIG. 4E is a cross-sectional view of the structure of FIG. 4D afterremoving the photolithographic mask and folming a first liner in theshallow trench,

FIG. 4F is a cross-sectional view of the structure of FIG. 4E afterforming a second liner in the shallow trench,

FIG. 4G is a cross-sectional view of the structure of FIG. 4F afterdepositing a blanket layer of insulating material to fill the shallowtrench,

FIG. 4H is a cross-sectional view of the structure of FIG. 4G afterplanarizing the surface of the structure to remove portions of theinsulating trench fill material and second liner material down to thepad nitride layer,

FIG. 4I is a cross-sectional view of the structure of FIG. 4H afterperforming a liner recess etch process to remove portions of the secondliner material present on the upper sidewalls of the shallow trench andform a recess region in an upper portion of the shallow trench,

FIG. 4J is a cross-sectional view of the structure of FIG. 4I afterremoving the pad nitride layer and depositing a layer of insulatingmaterial to fill the recess region within the shallow trench,

FIG. 4K is a cross-sectional view of the structure of FIG. 4J afterremoving the portion of the layer of insulting material down to the padoxide layer,

FIG. 4L is a cross-sectional view of the structure of FIG. 4K afterremoving the pad oxide layer, and

FIG. 4M is a cross-sectional view of the structure of FIG. 4L afterforming a gate dielectric layer on top of the substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described in furtherdetail with reference to STI structures for use with UTBB (ultra-thinbody and buried oxide) semiconductor substrates, which prevent defectmechanisms from occurring, such as the formation of electrical shortsbetween exposed portions of silicon layers on the sidewalls of shallowtrench of a UTBB substrate, in instances when trench fill material ofthe shallow trench is subsequently etched away and recessed below anupper surface of the UTBB substrate.

FIG. 1 is a cross-sectional view of a semiconductor device having a STIstructure according to one aspect of the invention. In general, thesemiconductor device 100 shown in FIG. 1 includes a semiconductorsubstrate 110 comprising a first silicon layer 112, a second siliconlayer 114, and a buried oxide layer 116 (BOX layer) disposed between thefirst silicon layer 112 and the second silicon layer 114. A high-k gatedielectric layer 120 is formed on the first silicon layer 112. A shallowtrench isolation structure 130 (STI structure) is formed in thesemiconductor substrate 110. The STI structure 130 comprises a shallowtrench 140 formed completely through the first silicon layer 112 andburied oxide layer 116, and partially through the second silicon layer114. The STI structure 130 further comprises a first liner 150, a secondliner 160, and a trench fill material 170.

In the device geometry of FIG. 1, the first liner 150 is formed onexposed portions of the first silicon layer 112 and second silicon layer116 on the sidewalls and bottom walls of the shallow trench 140. Thefirst liner 150 may be an oxide layer, such as an oxynitride layer, thatis grown on the exposed surfaces of the silicon layers 112 and 114 inthe shallow trench 140. The second liner 160 is conformally formed onthe sidewall and bottom surfaces of the shallow trench 140 on top of thefirst liner 150 and on the exposed surfaces of the BOX layer 116. Thetrench fill material 170 is formed over the second liner 160 filling theshallow trench 140. The trench fill material 170 may be formed of anysuitable insulating material such as silicon dioxide (SiO₂).

In accordance with principles of the invention, the second liner 160 ismade of a material that has a high etch selectivity with regard to thetrench fill material 170. For instance, the second liner 160 may be madeof a high-K dielectric material such as hafnium silicate and hafniumoxide. In this regard, the second liner 160 serves as a barrier, orotherwise provides etch stop protection, to protect the sidewallssurfaces of the shallow trench 140 and prevent defect mechanisms foroccurring (such as formation of shorts between exposed portions of thefirst silicon layer 112 and the second silicon layer 114 in the trench140) in instances when the trench fill material 170 is subsequentlyetched away and recessed below the upper surface of the substrate 110.

By way of specific example, after formation of the STI structure 130,the trench fill material 170 can be undesirably etched down (recessed)within the trench 140 as a result of one or more subsequent wet cleaningprocesses (wet etching with HF) that are performed as part ofpre-silicon epitaxy and/or pre-gate dielectric cleaning steps, forexample. In a UTBB device according to an aspect of the invention, thethickness of the first silicon layer 112 may be about 6 nm and thethickness of the BOX layer 116 may be in a range of 5-75 nm. In thisregard, as a result of the subsequent wet etching processes, the trenchfill material 170 can be recessed in the trench 140 to a level that isactually below the BOX layer 116. With conventional STI structures thatdo not include the protective liner 160 as shown in FIG. 1, thisrecessing of the trench fill material below the level of the BOX layer114 can expose portions of the first and second silicon layers 112 and114 within the trench, resulting in potential short defects between thefirst and second silicon layers 112 and 114.

For example, in semiconductor fabrication, a silicon epitaxialdeposition, or epitaxy, process can be performed to grow a thin layer ofsingle-crystal silicon over a single-crystal silicon substrate. Asilicon epitaxy process can be performed, for example, to selectivelygrow additional silicon in the source and drain regions of thesemiconductor substrate to increase the thickness of the first siliconlayer 112 outside the channel region to achieve lower resistance sourceand drain regions. However, during a silicon epitaxy process, if thefirst and second silicon layers 112 and 114 are exposed within thetrench 140 (due to the recessed trench fill material), the epitaxyprocess can cause silicon to grow on the sidewalls of the shallow trenchand cause an electrical (short) connection between the first and secondsilicon layers 112 and 114 of the substrate 110. Likewise, similardefect mechanisms can occur during a silicide process, when silicide isgrown on the surface of the first silicon layer 112 of the substrate 110in the source/drain regions to form low resistance source/draincontacts.

Moreover, these defect mechanisms can occur during subsequent BEOL (backend of line) processing, for example, when forming metallic via contactsto drain and source regions. In this process, a first layer ofinterlevel insulating material (e.g., oxide) is deposited over theactive surface of the semiconductor substrate and then via holes areetched in the first layer of insulating material down to devicecontacts. In instances where the via holes have some overlap with theSTI structures and the via holes are over-etched, the trench fillmaterial within the shallow trench can be also be etched away andrecessed down in the shallow trench such that the first and secondsilicon layers 112 and 114 are exposed on the sidewalls of the trench.During a subsequent via hole fill process, the metallic material fillingthe via hole can also fill in the upper part of the shallow trench andcause an electrical connection (short) between the exposed surfaces ofthe first and second silicon layers 112 and 114 within the shallowtrench.

To avoid these defect mechanisms, in accordance with principles of theinvention, the second liner 160 is employed to protect the sidewalls ofshallow trench 140 from being exposed due to subsequent processing stepsthat cause the trench fill material 170 to be etched away and recesseddown in the shallow trench 140. By forming the second liner 160 with amaterial that is more resistant to subsequent wet etching processes, forexample, the second liner 160 can serve as a protective barrier layerthat prevents exposure of the first and second silicon layers 112 and114 on the sidewalls of the shallow trench 140 when the trench fillmaterial 170 is over etched and recessed down below the BOX layer 116.

In the exemplary embodiment shown in FIG. 1, the second liner 160 isshown to be in contact with the high-k gate dielectric layer 120. Whenthe second liner 160 is made of a high-k dielectric material, the secondliner 160 can act as a conductive pathway for oxygen, in particular,oxygen found in the trench fill material 170 (e.g., SiO₂), causinghigh-K gate dielectric layer 120 to be contaminated with additionaloxygen atoms. This oxygen contamination of the gate dielectric layer 120can cause undesirable, random shifts in the threshold voltages of theFETs having gate structures formed by portions of the oxygencontaminated gate dielectric layer 120. Thus, in accordance with otheraspect of the invention, STI structures are provided to provideisolation between the second liner 160 and the gate dielectric layer120.

For instance, FIG. 2 is a cross-sectional view of a semiconductor devicehaving a STI structure according to another aspect of the invention,which provides isolation between the trench liner 160 and the gatedielectric layer 120. In particular, FIG. 2 shows a semiconductor device200 which is similar to the semiconductor device 100 shown in FIG. 1,except that the second liner 160 of the STI structure 130 is recesseddown below the surface of the substrate 110, thereby forming a recessregion 180 (or void) in the upper region of the shallow trench 140. Therecess region 180 provides isolation between the trench liner 160 andthe gate dielectric layer and prevents oxygen from diffusing from thetrench fill material 170 to the high-k gate dielectric layer 120 throughthe high-k trench liner 160. The recess region 180 is formed by etchingdown the portion of the second liner 160 in the upper region of the STIstructure 130 so that the gate dielectric layer 120 does not makecontact to the trench liner 160 when the gate dielectric layer 120 issubsequently formed. In the exemplary embodiment of FIG. 2, the trenchliner 160 is preferably recessed down any suitable depth that issufficient to prevent contact between the trench liner 160 and the gatedielectric layer 120, while providing the desired etch protective layeras discussed above to prevent exposure of the silicon layers 112 and 114on the sidewalls of the shallow trench 140.

FIG. 3 is a cross-sectional view of a semiconductor device having a STIstructure according to another aspect of the invention, which providesisolation between the trench liner 160 and the gate dielectric layer120. In particular, FIG. 3 shows a semiconductor device 300 which issimilar to the semiconductor device 200 shown in FIG. 2, except that therecess region 180 is filled with an insulating material (divot fill)190, such as silicon nitride or any other material, which does notconduct any appreciable amount of oxygen from the trench fill material170 to the high-K gate dielectric layer 120. The divot fill 190effectively isolates the second liner 160 and the high k gate dielectric120 and prevents diffusion of oxygen from the high-k trench liner 160 tothe high-K gate dielectric layer 120.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, 4J, 4K, 4L, and 4Mschematically illustrate methods for fabricating a semiconductor devicehaving STI structure according to aspects of the invention. Inparticular, FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, 4J, 4K, 4L and 4Mare cross-sectional views of portions of the semiconductor devices 100,200 and 300 of FIG. 1 at various stages of fabrication. FIG. 4A is across-sectional view of the semiconductor devices 100, 200 and 300 at aninitial stage of fabrication wherein a substrate 110 comprises a firstsilicon layer 112, a second silicon layer 114, and a BOX layer 116disposed between the first and second silicon layers 112 and 114. Thesubstrate 110 may be fabricated by forming an oxide layer (the BOX layer116) on a silicon substrate (the second silicon layer 114) and thenforming a thin silicon layer (the first silicon layer 112) on theinsulating layer 116. The first silicon layer 112 may have a thicknessin a range of about 5 nm to about 25 nm. The BOX layer 116 may have athickness in a range of about 5 nm to about 75 nm. The thickness of thesecond silicon layer 114 may be in a range of about 100 um to about 200um. In a UTBB design, the BOX layer 116 is sufficiently thin to allowthe second silicon layer 114 to be used as a back gate, while providinga sufficient insulation between the silicon layers 112 and 114.

A next step in the exemplary fabrication process comprises sequentiallyforming a pad oxide layer and a pad nitride layer over the substrate110. FIG. 4B is a cross-sectional view of the structure of FIG. 4A afterforming a pad oxide layer 200 on the substrate 110 and forming a padnitride layer 210 on the pad oxide layer 200. The pad oxide layer 200and the pad nitride layer 210 can be formed using well-known techniques.For instance, in one preferred embodiment, the pad oxide layer 200 isformed using a thermal oxidation process. The thickness of the pad oxidelayer 200 can be in a range of about 5 nm to about 8 nm. The pad nitridelayer 210 can be formed by depositing silicon nitride using a CVD(chemical vapor deposition) process such as LPCVD or PECVD. The padnitride layer 210 can be formed with a thickness in a range of about 20nm to about 80 nm. The pad oxide layer 200 and the pad nitride layer 210serve as sacrificial material layers in the exemplary fabricationprocess. The pad oxide layer 200 serves as a buffer layer between thepad nitride layer 210 and the first silicon layer 112.

A next step in the exemplary fabrication process is to form shallowtrenches in the substrate 110. In one aspect of the invention, theshallow trenches are formed using a standard photolithography process toform a lithographic mask and etch shallow trenches using thelithographic mask as an etch mask. FIG. 4C is a cross-sectional view ofthe structure of FIG. 4B after forming a photolithographic mask 220 (bypatterning a photoresist layer using known techniques) on the padnitride layer 210 which defines an opening 222 for etching a shallowtrench structure. FIG. 4D is a cross-sectional view of the structure ofFIG. 4C after etching the pad nitride and oxide layers and substrate toform a shallow trench 140. As shown in FIG. 4E, the shallow trench 140is formed in the substrate 110 by etching completely through the firstsilicon layer 112, the BOX layer 116 and partially through the secondsilicon layer 114. The etch process may be performed by sequentiallyetching various layers 210, 200, 112, 16 and 112 using dry plasmaetching or using any other etch processes and etch environments that arecommonly used to etch the materials forming the various layers 210, 200,112, 16 and 112.

After forming the shallow trenches 140, a next step in the exemplaryfabrication process is to remove the photolithographic mask 220 and linethe shallow trench 140 with an insulating material. FIG. 4E is across-sectional view of the structure of FIG. 4D after removing thephotolithographic mask 220 and forming a first liner 150 in the shallow140. The photolithographic mask 220 can be removed using dry plasmaoxygen ash process followed by a wet cleaning process. After removal ofthe photolithographic mask 220, the first liner 150 can be formed on theexposed surfaces of the first and second silicon layers 112 and 114 inthe shallow trench 140 using a thermal oxidation process to form a thinthermal oxide liner. The process of forming the first liner 150 can beperformed in a furnace or alternatively via a rapid thermal CVD tool, inthe presence of a nitrogen or oxygen environment. A thermal oxidationprocess will convert the exposed surfaces of the silicon layers 112 and114 into silicon dioxide, SiO₂. The silicon dioxide trench liner 150 canbe converted to an oxynitride liner if an ammonium (NH3) bake is usedafter the oxidation process. The oxynitride trench liner 150 protectsthe exposed surfaces of the silicon layers 112 and 114 in the shallowtrench 140. The thickness of the oxynitride trench liner 150 can be inthe range of about 0.5 nm to about 4 nm.

A next step in the exemplary fabrication process is to form a secondtrench liner on the sidewalls and bottom walls of the shallow trench140. FIG. 4F is a cross-sectional view of the structure of FIG. 4E afterconformally depositing a second layer of insulating material to form thesecond liner 160 in the shallow trench 140. The second liner 160 can bemade of any suitable material that has a high etch selectivity withregard to the oxide material that is subsequently used to fill theshallow trench 140. For instance, the second liner 160 may be formedwith material such as TiO2, HfO2, HfSiO, and HfSiON, any other high-Kdielectric/insulating materials. The second liner 160 may be depositedusing processes such as ALD (Atomic Layer Deposition) or MOCVD (MetalOrganic Chemical Vapor Deposition). As noted above, the second liner 160serves as an etch protection layer to protect the surfaces of the firstand second silicon layers 112 and 114 from exposure in the shallowtrench 140 in circumstances where a trench fill material is etched awayduring subsequent processing steps. The second liner 160 is made ofmaterial with high etch selectivity with respect to the trench fillmaterial, and does not etch away as fast as the trench fill materialduring etching/cleaning steps (e.g., hydrofluoric acid wet clean) thatwould etch away the trench fill material. In other embodiments, thesecond liner 160 may be nitridized to prevent uptake of oxygen.

After depositing the second liner 160 layer, the shallow trench 140 isfilled with an insulting trench fill material. FIG. 4G is across-sectional view of the structure of FIG. 4F after depositing ablanket layer of insulating material 170 to fill the shallow trench 140.The trench fill material 170 may be an oxide material, such as silicondioxide formed using an SACVD or spin coating process, or HDP (highdensity plasma) oxide. After the depositing the blanket layer of trenchfill material 170, the substrate surface is planarized down to the padnitride layer 210 to remove the portions of the trench fill material 170and second liner 160 on the surface of the substrate. FIG. 4H is across-sectional view of the structure of FIG. 4G after planarizing thesurface of the structure to remove portions of the insulating trenchfill material 170 and the second liner material 160 down to the padnitride layer 210. In another embodiment of the invention, thisplanarization process can be performed to planarize the surface of thestructure down to the pad oxide layer 200.

It is to be noted that the processing steps depicted in FIGS. 4A-4H areprocessing steps that can be sequentially performed as part of anexemplary process for fabricating each of the structures 100, 200 and300 depicted in FIGS. 1, 2 and 3. However, the sequence of additionalprocessing steps following the fabrication stage depicted in FIG. 4Hwill vary for the different structures depicted in FIGS. 1, 2 and 3. Forillustrative purposes, the additional process steps depicted in FIGS.4I˜4L show an exemplary sequence of processing steps for constructingthe structure 300 of FIG. 3.

In particular, FIG. 4I is a cross-sectional view of the structure ofFIG. 4H after performing a liner recess etch process to remove portionsof the second liner material 160 present on the upper sidewalls of theshallow trench 140 and form a recess region 180 in an upper portion ofthe shallow trench 140. The liner recess etch process may be performedusing a dry etch process using a BCl₃ dry chemistry. The second liner160 may be recessed down to a level that does not extend past a bottomof the BOX layer 116. The recess etch process is preferably highlyselective to the trench fill material 170 and the pad nitride layer 210so that the trench fill material 170 is not significantly etched(although the pad nitride layer 210 can serves as a sacrificial layer inthis process).

In a next phase of fabrication, FIG. 4J is a cross-sectional view of thestructure of FIG. 4I after removing the pad nitride layer 210 andconformally depositing a layer of insulating material 190 to fill therecess region 180 within the shallow trench 140. The pad nitride layer210 can be removed with a wet etch process using hydrophosphoric acid.The layer of insulating material 190 may be a layer of silicon nitrideor any other material which does not conduct appreciable amount ofoxygen. The layer of insulating material 190 may be conformallydeposited using know techniques to fill the recess region 180. As notedabove, the silicon nitride divot fill 190 isolates the second liner 160and trench fill material 170 from a subsequently formed high-K gatedielectric layer and, thus prevents the flow of oxygen atoms to thehigh-K gate dielectric layer.

FIG. 4K is a cross-sectional view of the structure of FIG. 4J afterremoving the portion of the layer insulting material 190 (divot fillmaterial) down to the pad oxide layer 200. The excess divot fillmaterial may be removed from the top horizontal surface of the substrateusing a dry directional etch back process or using a wet etch process toremove the divot fill material 190 selective to the trench fill material170 and pad oxide layer 200.

In a next phase of fabrication, FIG. 4L is a cross-sectional view of thestructure of FIG. 4K after removing the pad oxide layer 200. In thisprocess, the pad oxide layer 200 can be removed using any suitable oxideetch process. This etch process may result in some etching and recess ofthe trench fill material 170. However, the divot fill 190 and secondtrench liner 160 serve to protect the sidewalls of the shallow trench140 in the event that the trench fill material 170 is etch and recessedwith the pad oxide removal process and other etch/clean processingsteps.

4M is a cross-sectional view of the structure of FIG. 4L after forming agate dielectric layer 120 on top of the substrate surface. In one aspectof the invention, the gate dielectric layer 120 may formed using knownmaterials and techniques. For instance, the gate dielectric layer 120may comprise a stack of materials comprising a first, thin interfaciallayer of dielectric/insulating material formed on the first siliconlayer 112 and a second layer of high dielectric material formed on thefirst interfacial layer As discussed above, the divot fill 190 isolatesthe second liner 160 from the high k gate dielectric layer 120 toprevent diffusion of oxygen from the second liner 160 to the high k gatedielectric layer 120 and 210 and thereby prevent a significant shift inthe threshold voltages caused by oxygen contamination of the high k gatedielectric layer 120.

As noted above, the processing steps for constructing the structures 100and 200 of FIGS. 1 and 2 will vary. For instance, with regard to thestructure of FIG. 1, starting from the stage of fabrication depicted inFIG. 4G, a combination of etch and/or planarizing processes can beperformed to remove the layers of trench fill material 170, the secondliner 160, the pad nitride 210, and the pad oxide layer 200 down to thefirst silicon layer 112. Thereafter, a gate dielectric layer 120 can beformed as discussed above with reference to FIG. 4L. The resultingstructure 100 is depicted in FIG. 1, wherein the second trench liner 160would be in contact with the gate dielectric layer 120.

Moreover, with regard to the structure 200 of FIG. 2, starting from thestage of fabrication depicted in FIG. 4G, a combination of etch and/orplanarizing processes can be performed to remove the layers of trenchfill material 170, the second liner 160, and the pad nitride 210 down tothe pad oxide layer 200. Thereafter, a recess process such as discussedabove with reference to FIG. 4I could be performed to recess the secondliner material 160 to form a recess region 180. This liner recessprocess could then be followed by a pad oxide removal and formation ofthe gate dielectric layer 120 as discussed above with reference to FIG.4L. The resulting structure 200 is depicted in FIG. 2, wherein thesecond trench liner 160 is isolated from the gate dielectric layer 120by the recess region 180.

It is to be understood that the invention is not limited to theparticular materials, features, and processing steps shown and describedherein. Modifications to the illustrative embodiments will becomeapparent to those of ordinary skill in the art. It should also beunderstood that the various layers and/or regions shown in theaccompanying figures are not drawn to scale, and that one or moresemiconductor layers and/or regions of a type commonly used in suchintegrated circuits may not be explicitly shown in a given figure forease of explanation. Particularly with respect to processing steps, itis to be emphasized that the descriptions provided herein are notintended to encompass all of the processing steps that may be requiredto form a functional integrated semiconductor device. For convenience ofexplanation and not intended to be limiting, the semiconductor devicesand methods of fabrication are described herein for siliconsemiconductors but persons of ordinary skill in the art will understandthat other semiconductor materials can also be used.

Further aspects of the present invention provide STI structures that canbe utilized in integrated circuit chips with various analog and digitalintegrated circuitries. In particular, integrated circuit dies can befabricated having STI structures formed in UTBB substrates to isolateactive regions for semiconductor devices such as field-effecttransistors, bipolar transistors, metal-oxide-semiconductor transistors,diodes, resistors, capacitors, inductors, etc., forming analog and/ordigital circuits. An integrated circuit in accordance with the presentinvention can be employed in applications, hardware, and/or electronicsystems. Suitable hardware and systems for implementing the inventionmay include, but are not limited to, personal computers, communicationnetworks, electronic commerce systems, portable communications devices(e.g., cell phones), solid-state media storage devices, functionalcircuitry, etc. Systems and hardware incorporating such integratedcircuits are considered part of this invention. Given the teachings ofthe invention provided herein, one of ordinary skill in the art will beable to contemplate other implementations and applications of thetechniques of the invention.

Although the exemplary embodiments of the present invention have beendescribed herein with reference to the accompanying figures, it is to beunderstood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade therein by one skilled in the art without departing from the scopeof the appended claims.

What is claimed is:
 1. A method of forming a semiconductor device,comprising: forming a high-k gate dielectric layer on a semiconductorsubstrate comprising a first silicon layer, a second silicon layer, anda buried oxide layer disposed between the first silicon layer and thesecond silicon layer; and forming a shallow trench isolation structureformed in the semiconductor substrate, wherein forming a shallow trenchisolation structure comprises: forming a shallow trench in the substratethrough the first silicon layer, the buried oxide layer and partiallythrough the second silicon layer; forming a first liner conformallylining the shallow trench; and filling the shallow trench with a trenchfill material, wherein the first liner is formed of a material havingetch selectivity with regard to the trench fill material, whereinforming the shallow trench isolation structure further comprises etchingthe first liner to recess the first liner down in the shallow trench andcreate a void region disposed between an upper recessed surface of thefirst liner and an upper surface of the first silicon layer, wherein thevoid region isolates the first liner from the high-k gate dielectriclayer formed on the first silicon layer.
 2. The method of claim 1,wherein the first liner is a high-k dielectric liner.
 3. The method ofclaim 1, wherein forming the shallow trench isolation structure furthercomprises forming a second liner directly on exposed surfaces of thefirst and second silicon layers in the shallow trench, prior to formingthe first liner.
 4. The method of claim 1, wherein the second liner isformed of oxynitride.
 5. The method of claim 1, wherein a thickness ofthe buried oxide layer is in a range of about 5 nm to about 75 nm. 6.The method of claim 1, wherein the trench fill material comprises anoxide material.
 7. A method of forming a semiconductor device,comprising: forming a high-k gate dielectric layer on a semiconductorsubstrate comprising a first silicon layer, a second silicon layer, anda buried oxide layer disposed between the first silicon layer and thesecond silicon layer; and forming a shallow trench isolation structureformed in the semiconductor substrate, wherein forming a shallow trenchisolation structure comprises: forming a shallow trench in the substratethrough the first silicon layer, the buried oxide layer and partiallythrough the second silicon layer; forming a first liner conformallylining the shallow trench; and filling the shallow trench with a trenchfill material, wherein the first liner is formed of a material havingetch selectivity with regard to the trench fill material, etching thefirst liner to recess the first liner down in the shallow trench andcreate a void region disposed between an upper recessed surface of thefirst liner and an upper surface of the first silicon layer; and fillingthe void region with an insulating material, wherein the insulatingmaterial isolates the first liner from the high-k gate dielectric layerformed on the first silicon layer.
 8. The method of claim 7, wherein thefirst liner is a high-k dielectric liner.
 9. The method of claim 7,wherein forming the shallow trench isolation structure further comprisesforming a second liner directly on exposed surfaces of the first andsecond silicon layers in the shallow trench, prior to forming the firstliner.
 10. The method of claim 9, wherein the second liner is formed ofoxynitride.
 11. The method of claim 7, wherein the insulating materialis a material that does not conduct an appreciable amount of oxygen. 12.The method of claim 7, wherein the insulating material comprises siliconnitride.
 13. The method of claim 7, wherein a thickness of the buriedoxide layer is in a range of about 5 nm to about 75 nm.
 14. The methodof claim 7, wherein the trench fill material comprises an oxidematerial.